Atomic Theory Notes

Atomic Theory
Democritus
 400
B.C.
 Earth
 Air
 Fire
 Water
Greek “Thinkers”

1.
2.
3.
Atoms – “indivisible” – smallest part of
an element that retains the property of
that element
All atoms have a definite size
(“incompressible”)
All atoms are neutral
All matter is composed of atoms too
small to be seen which cannot be divided.
Greek “Thinkers”
4.
5.
6.
There is empty space between atoms
Atoms are completely solid
 Later proved incorrect
Atoms are different sizes, shapes, and
weights
Alchemists
 Middle
Ages
 Spent most of their time trying to
change metals into silver and gold
 Transmutation – changing one
element into another
 Added knowledge about the properties
of elements
Antoine Lavoisier
1789
 Able to list 33 known
elements
 Law of Conservation of
Mass
 “Recycling” Law
 Matter can be changed;
BUT it cannot be created
or destroyed.

John Dalton
 1803
 “The
Father of
Modern Atomic
Theory”
 Used all former ideas
to come up with his
atomic theory
John Dalton
1.
2.
3.
Each element consists of individual
atoms
All atoms of the same element are
identical in mass and properties
All atoms of different elements are
different in mass and properties
John Dalton
4.
5.
Atoms combine with each other in
specific ratios to make compounds
Chemical reactions occur when
atoms are separated, joined, or
rearranged, but the atoms of one
element are not changed into the
atoms of another by chemical
reaction
 1869
 Russian
Dmitri Mendeleev
Scientist
 Devised the Periodic Table
 Arranged the known
elements by atomic weight
and noticed that properties
re-occurred periodically
 Left blanks for undiscovered
elements
400-1890’s

Atom was viewed as solid and indivisible
Marie & Pierre Curie
Henri Becquerel
 Discovered
Radioactivity
 Polonium (Po), Radium (Ra), Uranium (U)
 Smaller particles are emitted from the atom
 Gamma Rays – neutral
 Beta rays – negative (electron)
 Alpha rays – positive (2 protons & 2 neutrons)
J.J. Thomson

Discovered the electron using the cathode ray tube

Electrons are NEGATIVE particles

e- are very small in weight
Plum pudding model
Later found positive particles


(protons)
Robert Millikan
 Determined
 (9.11
 This
the mass of the e-
x 10-31 kg)
mass is MUCH smaller than the
mass of the proton
Ernest Rutherford

Gold foil experiment
Found an atom has a very small positive nucleus
surrounded by tiny moving e and lots of empty
space
Modern Gold Foil
 Protons must be in center
Experiment

James Chadwick
 Discovered
 No
the neutron
charge
 Same weight as proton
Atomic Structure
 Subatomic
Particles
protons (p+) = positive
electrons (e-) = negative
neutrons (n0) = neutral
Atomic Structure
 Atomic
number = # p+
 Bromine
A.N.
= 35
35 protons

Protons determine the element
Atomic Structure
 Atoms
are neutral
protons (p+) = # electrons (e-)
 Bromine
#
a.n.
= 35
35 protons (+)
Therefore, 35 electrons (-)
Atomic Structure
 Atomic
 The
Mass
weight of the atom is in the nucleus
 Protons and Neutrons in the nucleus
 Atomic mass = #p+ + #n0
 Bromine a.m. = 80
 80 = 35 + #n0
 #n0 = 45
Atomic Structure
 Atoms
are neutral
# protons (p+) = # electrons (e-)
Atomic number = # p+
Atomic mass = #p+ + #n0
= neutrons
= protons
C-14
C-12
Nucleus
Nucleus
= neutrons
= protons
C-14
C-12
Nucleus
Nucleus
-Both are Carbon (both have 6 protons)
-Have different number of neutrons!
Isotopes
 Atoms
of the same element (same
atomic number) with different atomic
masses (different # of neutrons).
C-14 and C-12